Playing as the Legions (Disciples 2).

Sometimes I would summon a demon, but not a tiamat, but its neighbor with a burn effect.

He's also one of my favorites. I think the burn effect is better than paralysis. It deals more damage.

I often summoned and leveled up a gargoyle.

I always take a gargoyle with me; an archer is never unnecessary. The only problem is that neutrals seem to love it even more than possessed creatures; they always attack it, and even armor doesn't always help.

An incubus is the best choice, without a doubt.

I agree. I use a hero who is an archdevil, so I don't need a second fire mage. A doppelganger is too specific and not suitable for long-term development, and a succubus is useless – it often misses, and everyone has immunity to Mind magic. It's a different story when the hero deals 200 damage; the incubus doesn't always have time to take a turn.